For both users and operators of wireless communications systems it is often desirable to locate subscriber units that are making calls, or are otherwise transmitting or receiving data. Applications for technology that can locate subscribers include locating subscribers requesting 911-emergency services, assisting law enforcement agencies in various law enforcement activities, providing users with local area maps or directions, and providing users with advertising information related to the user's location.
Several technologies have been either used or proposed for providing the subscriber unit's location in a communication system, and each of these technologies has disadvantages. For example, a time difference of arrival (TDOA) system uses measurements from a plurality of base transceivers at known locations. This type of system requires signal reception of the subscriber unit's signal at more than one base transceiver, time synchronization between the base transceivers, and the measurement of line-of-sight wave front propagation, without confusing the line-of-sight signals with multipath signals, which are signals bounced and diffracted from objects affecting the propagation of the signal. The accuracy of synchronization of base transceivers in a cellular system based on interim specification 95 (IS-95), a code division multiple access (CDMA) standard, is not accurate enough to make signal propagation measurements that will provide location estimates with the desired resolution. Additionally, in a dense downtown area, also known as the urban canyon, detection of the line-of-sight wave front may not be possible because line-of-sight signal components may be shadowed or obscured by close-in multipath components.
More advanced TDOA systems use smart antennas to help eliminate some of the multipath components, enhancing detection of the prompt wave front. Smart antennas may also be able to add angle-of-arrival (AOA) measurements to the geometric equations for location finding, which further enhances performance. Unfortunately, the multipath environment of some urban canyons is too confused for smart antennas to be an effective solution. Smart antenna overlap solutions also require adding special phased array antennas to work in conjunction with the existing antennas. Service providers will most likely be reluctant to invest in additional antennas until smart antennas are needed for capacity requirements as well as location finding requirements.
Some engineers and operators propose using the global positioning system satellite system for determining a subscriber's location. This technology also has a problem in the urban canyon. In many places it is not possible to receive line-of-sight signals from a number of satellites that is needed to produce an accurate location estimate.
Other proposals include placing many microcells in an urban core area so that the subscriber's location is estimated by identifying what cell site the subscriber is using. Although microcells are gaining popularity in certain areas, the market has generally rejected microcells for capacity coverage, which means that the number of microcells deployed is probably not extensive enough to support location finding. Most current communication deployment plans still specify macrocellular coverage.
Therefore, it should be apparent that there remains a need for an improved, cost-efficient approach for locating subscriber units in a cluttered area of a communications system service area, where radio frequency signal characteristics of indirect or non-line-of-sight propagation signals are beneficially used to locate a subscriber unit. Accordingly, there is also a need for an improved method and system for comparing measured radio frequency signal propagation characteristics in a wireless communication system, wherein the radio frequency signature is responsive to the dominant features of radio frequency propagation characteristics of a signal transmitted from a selected signature region.